1. Field of the Invention
The present invention relates to the technical field of operational amplifiers and, more particularly, to an operational amplifying device with auto-adjustment output impedance.
2. Description of Related Art
The output of an operational amplifier typically has a frequency compensation to stabilize its closed-loop gain. Namely, a compensation resistor Rf is added to the output of the operational amplifier to increase the phase margin (PM). FIG. 1 is a schematic diagram of a typical operational amplifier with a compensation resistor. As shown in FIG. 1, the transfer function of the closed-loop voltage gain can be expressed as:
                    Av        =                ⁢                              Vout            ⁢                                                  ⁢            1                                Vin            ⁢                                                  ⁢            1                                                  =                ⁢                              Vout                          Vin              ⁢                                                          ⁢              1                                ×                                    Vout              ⁢                                                          ⁢              1                        Vout                                                  =                ⁢                  A          ×                                                    R                ⁢                                                                  ⁢                1                            +                              1                                  jω                  ⁢                                                                          ⁢                  C                  ⁢                                                                          ⁢                  1                                                                    Rf              +                              R                ⁢                                                                  ⁢                1                            +                              1                                  jω                  ⁢                                                                          ⁢                  C                  ⁢                                                                          ⁢                  1                                                                                            =                ⁢                  A          ×                                    1              +                              jω                ⁢                                                                  ⁢                R                ⁢                                                                  ⁢                1                ⁢                C                ⁢                                                                  ⁢                1                                                    1              +                                                jω                  ⁡                                      (                                          Rf                      +                                              R                        ⁢                                                                                                  ⁢                        1                                                              )                                                  ⁢                C                ⁢                                                                  ⁢                1                                                                            =                ⁢                  A          ×                                                    1                +                                  f                                      f                                          Z                      ⁢                                                                                          ⁢                      1                                                                                                  1                +                                  f                                      f                                          P                      ⁢                                                                                          ⁢                      1                                                                                            .                              
From the equation, it is known that the transfer function has one zero (with frequency fZ1) and one pole (with frequency fP1), which are
      1          2      ⁢      π      ⁢                          ⁢      R      ⁢                          ⁢      1      ⁢      C      ⁢                          ⁢      1        and            1              2        ⁢                  π          ⁡                      (                          Rf              +                              R                ⁢                                                                  ⁢                1                                      )                          ⁢        C        ⁢                                  ⁢        1              ,  respectively.
FIG. 2 shows a Bode plot of a large compensation resistor Rf, and FIG. 3 shows a Bode plot of a small compensation resistor Rf. As shown in FIG. 2, P1 and Z1 indicate a pole and a zero of RC output net in FIG. 1, respectively, and P2 and P3 indicate poles of the operational amplifier in FIG. 1, assuming that the zero and pole frequencies thereof have a relation of fP1<fP2<fZ1<fP3. When a large resistor Rf is selected, as shown in FIG. 2, it can be seen that the frequency of P1 becomes lower, so that only one pole P1 is present before 0 dB. From FIG. 2, PM=70° can be found, but the output Vout1 becomes slower. When a small resistor Rf is selected, as shown in FIG. 3, the frequency of P1 becomes higher so that only two poles P1 and P2 are present before 0 dB. From FIG. 3, PM=45° can be found, but the output Vout1 becomes faster, and the overshoot and undershoot may occur.
It can be observed from FIGS. 2 and 3 that, when a large resistor Rf is designed, the phase margin PM is larger while encountering a problem in that the output Vout1 is slower, and when a small resistor Rf is designed, the output Vout1 becomes faster, while encountering a problem in that the phase margin PM becomes smaller and the overshoot and undershoot may easily occur. For a design requirement of fast output and high stability, it is difficult to use such a resistor Rf compensation phase way to achieve the requirement. In addition, when resistor Rf is getting smaller, the operational amplifier requires additional Miller compensation or another compensation to increase the stability.
Therefore, it is desirable to provide an improved operational amplifier device to mitigate and/or obviate the aforementioned problems.